The present invention relates to a process and new sorbents for selective adsorption and recovery of alkenes from gaseous mixtures containing the alkenes.
Several methods are known for the separation of selected organics from gaseous mixtures. These include, for example, cryogenic distillation, liquid adsorption, membrane separation and pressure swing adsorption in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. In an analogous method, temperature swing adsorption is used in which adsorption occurs at a lower temperature than the temperature at which the adsorbent is regenerated. In these adsorption techniques, after adsorption occurs, release of the adsorbed material is achieved by either decreasing the pressure or raising the temperature. Of these methods, cryogenic distillation and liquid adsorption represent commonly known methods for separating selected organics from gaseous mixtures. Cryogenic distillation has been used for over sixty years for separation. However, it is very energy intensive and difficult to accomplish because of relatively close volatilities when alkene alkane (olefin paraffin) separation is required. For example, ethane ethylene separation is carried out at about xe2x88x9225xc2x0 C. and 320 psig (2.603 MPa) in a column containing over 100 trays, and propane propylene separation is performed by an equally energy-intensive distillation at about xe2x88x9230xc2x0 C. and 30 psig (0.308 MPa). It is evident that high capital costs and high operational costs are incurred in any cryogenic distillation approach. Early attempts were made to use liquid solutions for separation by means of metallic ions dispersed in solution. Such methods are very difficult to conduct and not easily adaptable to commercial use.
More recently, molecular sieve zeolites have been investigated to selectively adsorb carbon monoxide and hydrocarbons from gaseous mixtures. However, these zeolites have shown only moderate capacity for recovery of the targeted compound to be adsorbed. An example is adsorption on cuprous ion exchanged y-type zeolites (U.S. Pat. Nos. 4,717,398 and 5,365,011). Presently, many of the available adsorbents known for other uses, such as carbon monoxide removal, do not have selectivities for olefins as demonstrated by the aforesaid ""398 and ""011. Therefore, what is needed are new adsorbents (sorbents) effective for olefin/paraffin separation.
The invention provides new adsorbents for use in separating selected gaseous hydrocarbons from a mixture comprising the hydrocarbons. The invention provides new methods for accomplishing such separation using the novel adsorbents. The new adsorbents are very effective for selective adsorption of alkenes such as ethylene, propylene, and mixtures of these from a gaseous mixture which comprises the alkene. In one embodiment, the invention provides an adsorbent for preferential adsorption which comprises a metal compound supported on a carrier. The metal compound is characterized by the ability to releasibly retain the alkene whereby the alkene is preferentially adsorbed from the mixture. This produces a non-adsorbed component and an alkene-rich adsorbed component. Next, by changing at least one of pressure and temperature, the alkene-rich component is thereby released from the adsorbent.
The adsorbent preferably comprises a metal compound selected from a silver compound, a copper compound and mixtures thereof. The preferred carrier comprises a plurality of pores having a pore size greater than the molecular diameter of the alkene. The compound of silver or copper is characterized by formation of xcfx80-complexation bonds between the silver or copper and the alkene for accomplishing the retention of the alkene by the adsorbent. When pressure and/or temperature is changed, the silver or copper compound releases the alkene-rich component from the adsorbent.
The metal compound is preferably a salt selected from acetate, benzoate, bromate, chlorate, perchlorate, chlorite, citrate, nitrate, nitrite, sulfate and halide (F, Cl, Br, I) and mixtures of these. The preferred silver salt is silver nitrate. Other salts of silver are as per the group defined above where the preferred halide is fluoride. Another preferred adsorbent is salt of copper selected from the group consisting of bromide, fluoride, iodide and sulfates, supported on a carrier.
The carrier is a high surface area support selected from refractory inorganic oxide, molecular sieve, activated carbon, pillared clay, and mixtures of these. The carriers are preferably characterized by a BET surface area greater than about 50 square meters per gram and up to about 2,000 square meters per gram and comprise a plurality of pores having a pore size greater than about 3 angstroms and up to about 10 microns. Preferably, the adsorbent comprises finely divided particles of silica with silver nitrate dispersed on and supported on the particles.
Preferential adsorption is achieved at a pressure greater than the desorption (release) pressure. Preferential adsorption pressure may be as high as about 35 atmospheres or more; and the desorption pressure may be as low as sub-atmospheric, significant vacuum, 0.01 atmosphere or less. The pressure of preferential adsorption is in a range of about 1 to about 35 atmospheres; desirably about 1 to 10 atmospheres; and most desirably about 1 to about 2 atmospheres. The pressure of release is in a range of about 0.01 atmospheres to about 5 atmospheres; and
desirably in a range of about 0.1 atmospheres to about 0.5 atmospheres.
The temperature of preferential adsorption is conveniently selected to be in a range of about 0xc2x0 C. to about 50xc2x0 C.; and desirably in a range of about 25xc2x0 C. to about 50xc2x0 C. The temperature of release is selected to be in a range of about 70xc2x0 C. to about 200xc2x0 C.; and desirably 100xc2x0 C. to about 120xc2x0 C.
There are several methods for accomplishing dispersion of the metal compound onto a carrier or support.
These methods include thermal monolayer dispersion, impregnation by incipient wetness technique, and spray application. In a preferred embodiment, the adsorbent of the invention is prepared by dispersing the metal compound onto the carrier under conditions that do not decompose or oxidize the compound. It is preferred that the dispersion permits the chemical compound to retain its character. The impregnation by incipient wetness technique accomplishes these objectives. This technique avoids decomposition of the metal compound which is more likely to occur by thermal dispersion heating method. Further, incipient wetness technique permits the effective dispersion of a monomolecular layer of metal compound on the carrier.
The invention provides substantial advantages over conventional methods for separating components of a gaseous mixture due to the effective and economical process and adsorbents provided by the invention.
Objects, features, and advantages of the invention include an improved method for separating olefins from paraffins. Another object is to provide new adsorbents for use in such separation.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiment, claims, and accompanying drawings.